In the United States and around the world, the demand for electrical power continues to grow. At the same time, the public power grid remains subject to occasional failures, even in the United States. These problems with the capacity and reliability of the public power grid have driven the development of distributed energy resources (DER), small independent power generation and storage systems which may be owned by, and located near, consumers of electrical power.
One motivating factor is that distributed energy resources can provide more reliable power in critical applications, as a backup to the primary electrical supply. For example, an interruption of power to a hospital can have life-threatening consequences. Similarly, when power to a factory is interrupted, the resulting losses, for example in productivity, wasted material in process that must be scrapped, and other costs to restart a production line, can be catastrophic. In situations like these, where the loss of electrical power can have serious consequences, the cost of implementing a distributed energy resource as a backup can be justified.
Reliability is not the only factor driving the development of distributed energy resources. Power from a distributed energy resource can, in some cases, be sold back to the main power grid. Geographically distributed sources of power, such as wind, solar, or hydroelectric power, may be too limited or intermittent to be used as the basis for a centralized power plant. By harnessing these types of geographically distributed sources using multiple distributed energy resources, these types of power sources can supplement or replace conventional power sources, such as fossil fuels, when the main power grid is available, and provide backup to their owners when the main power grid is unavailable.
In this context, distributed energy resources (DER) have emerged as a promising option to meet current and future demands for increasingly more reliable electric power. Power sources for DER systems, sometimes called “microsources,” range in size and capacity from a few kilowatts up to 10 MW. These power sources may include a variety of technologies, both supply-side and demand-side, and they are typically located where the energy is used. The term “microgrid” is sometimes used for an electrical circuit comprising a power bus, at least one distributed energy resource, and at least one accompanying load on the power bus.
Microgrids and distributed energy resources are usually adapted to operate in either “island” mode or “grid” mode. An interface switch connects the utility grid and the power bus of the microgrid and so the interface switch handles the transitions between island and grid mode and vice-versa. When the microgrid operates in island mode, the interface switch is in an open condition so that the microgrid is electrically isolated from the utility grid. When the microgrid operates in grid mode, the interface switch is in a closed condition so that the microgrid is electrically connected to the utility grid.
Generally, the interface switch has the task of disconnecting all feeders with distributed energy resources, DER, from the grid during protection, IEEE 1547 and power quality events. The interface switch thus plays a key role in the interface between the microgrid and the utility system, responsible for the successful handling of all utility/microgrid interconnection issues and concerns.
Thus, there is a need for an interface switch, and related methods, to enable islanding and re-connection of distributed energy resources in a seamless and automatic manner that minimizes any undesirable voltage transients, current surges, or other undesirable transients or strains on the system. What is further needed is an interface switch, and related methods, that can operate using only locally available information, such as AC voltage magnitude or phase measurements, that can be obtained locally at the terminals of the switch. What is further needed is a simplified interface switch, and related method, that depends only on local measurement of the envelope of the voltage difference across the interface switch for operation.